1. Lec 9 Subnet 3.1 Computer Networks Al-Mustansiryah University
Elec. Eng. Department College of Engineering Fourth Year Class
Lec 9
Subnet
3.1

2. Note
The first address in a block is normally not assigned to any device; it is used as the network address that represents the organization to the rest of the world.
3.2

3. Subnetting – Example 3.3 Host IP Address: 138.101.114.250
Network Mask:
Subnet Mask:
Given the following Host IP Address, Network Mask and Subnet mask find the following information:
Major Network Information
Major Network Address
Major Network Broadcast Address
Range of Hosts if not subnetted
Subnet Information
Subnet Address
Range of Host Addresses (first host and last host)
Broadcast Address
Other Subnet Information
Total number of subnets
Number of hosts per subnet
3.3

4. Major Network Information
Host IP Address:
Network Mask:
Subnet Mask:
Major Network Address:
Major Network Broadcast Address:
Range of Hosts if not Subnetted: to
3.4

5. Step 1: Convert to Binary
Step 1:
Translate Host IP Address and Subnet Mask into binary notation
3.5

6. Step 2: Find the Subnet Address
Determine the Network (or Subnet) where this Host address lives:
1. Draw a line under the mask
2. Perform a bit-wise AND operation on the IP Address and the Subnet Mask
Note: 1 AND 1 results in a 1, 0 AND anything results in a 0
3. Express the result in Dotted Decimal Notation
4. The result is the Subnet Address of this Subnet or “Wire” which is
3.6

7. Step 3: Subnet Range / Host Range
Determine which bits in the address contain Network (subnet) information and which contain Host information:
Use the Network Mask: and divide (Great Divide) the from the rest of the address.
Use Subnet Mask: and divide (Small Divide) the subnet from the hosts between the last “1” and the first “0” in the subnet mask.
3.7

8. Step 4: First Host / Last Host
Host Portion
Subnet Address: all 0’s
First Host: all 0’s and a 1
Last Host: all 1’s and a 0
Broadcast: all 1’s
3.8

9. Step 5: Total Number of Subnets
Number of subnet bits 10
210 = 1,024
1,024 total subnets
Subtract one “if” all-zeros subnet cannot be used
Subtract one “if” all-ones subnet cannot be used
1,022 total subnets
3.9

10. Step 6: Total Number of Hosts per Subnet
Number of host bits 6
26 = 64
64 host per subnets
Subtract one for the subnet address
Subtract one for the broadcast address
62 hosts per subnet
3.10

11. Example 3.10
An ISP is granted a block of addresses starting with /16 (65,536 addresses). The ISP needs to distribute these addresses to three groups of customers as follows:
a. The first group has 64 customers; each needs addresses.
b. The second group has 128 customers; each needs addresses.
c. The third group has 128 customers; each needs addresses.
Design the subblocks and find out how many addresses are still available after these allocations.
3.11

12. Example 3.10 (continued) Solution Figure 3.9 shows the situation.
Group 1
For this group, each customer needs 256 addresses. This means that 8 (log2 256) bits are needed to define each host. The prefix length is then 32 − 8 = 24. The addresses are
3.12

13. Example 3.10 (continued) Group 2
For this group, each customer needs 128 addresses. This means that 7 (log2 128) bits are needed to define each host. The prefix length is then 32 − 7 = 25. The addresses are
3.13

14. Example 3.10 (continued) Group 3
For this group, each customer needs 64 addresses. This means that 6 (log264) bits are needed to each host. The prefix length is then 32 − 6 = 26. The addresses are
Number of granted addresses to the ISP: 65,536
Number of allocated addresses by the ISP: 40,960
Number of available addresses: 24,576
3.14

15. Figure 3.9 An example of address allocation and distribution by an ISP
3.15

16. Table 3.3 Addresses for private networks
3.16

17. Solution: Net Sub Host 24 Bit 3 Bit 8 Bit
Example 3.11 :A company is granted the site address The company needs six subnets. Design the subnets?
Solution:
No. of subnet must be power of 2 therefore we design 8 subnets
No.of subnet bits=Log2(8)=3 bits
Ip address is class c
Net Sub Host
24 Bit Bit Bit
3.17

18. 3.18 Subnet NET . Subnet . Host Subnet IP Subnet 0 211.80.64 000 00000
11111
Subnet 1
001
Subnet 2
010
Subnet 3
011
Subnet 4
100
Subnet 5
101
Subnet 6
110
Subnet 7
111
3.18

19. Broadcast Address
(Directed Broadcast)
Purpose: This figure explains how broadcast addresses work.
Emphasize: A range of addresses is needed to allocate address space. A valid range of addresses is between subnet zero and the directed broadcast.
The following RFCs provide more information about broadcasts:
RFC 919, Broadcasting Internet Datagrams
RFC 922, Broadcasting IP Datagrams in the Presence of Subnets
Cisco’s support for broadcasts generally complies with these two RFCs. It does not support multisubnet broadcasts that are defined in RFC 922. X
(Local Network Broadcast)
(All Subnets Broadcast)
3.19